Fix bug #2176: autochk_interval granularity is low
This also fixes Debian bug “Please increase mail auto check period.”
See: https://bugs.debian.org/712758
Former bug wanted sub-minute granularity and latter the opposite: just
check a few times a day, so this allows user to specify also hours and
seconds in addition to existing minutes to address both requests.
Checking range was from 1 minute to 1 hour 40 minutes and now is
extended to a range from 10 seconds to 100 hours minus one second.
Default for new configurations is preserved (10 minutes).
Configuration version is raised to 2 for migrating existing values
from minutes to seconds.